1. Field of the Invention
The invention relates to an optical fiber cable comprising a component of polymeric material having, in particular, an improved dimensional stability and to said optical fiber cable component. More in particular, said cable component is preferably in the form of a tube of polymeric material comprising at least one optical fiber housed therein.
2. Description of the Related Art
As mentioned in U.S. Pat. No. 5,911,023, in designing the cable structure, it is important to ensure that process or construction induced stresses related to cable production do not interfere with optical fiber performance. For extruded components, such as buffer tubes containing optical fibers, poor flexibility, compression-tension resistance and thermal expansion properties may result in an overall reduced efficiency of the optical fibers and residual stress in the finished product.
According to the above cited patent, while fiber optic buffer tubes have been primarily made from engineering resins (such as polybutylen-terephtalate (PBT), polycarbonate, polyamides), the use of polyolefin buffer tubes have become increasingly attractive.
For instance, U.S. Pat. No. 5,574,816 discloses a buffer tube for an optical fiber cable made from a propylene-ethylene copolymer resin having nucleating agents and filler materials disbursed therein. The nucleating agents and filler materials improve compression-tension resistance and thermal expansion properties of the polypropylene-polyethylene buffer tube; further, a low shrinkage, without negatively affecting flexibility and cost, is observed. Examples of materials suitable for realising the buffer tubes are Stamylan 83E10N, a polypropylene resin produced by DSM Engineering Plastics of Geleen, The Netherlands, having approximately 11 weight percent polyethylene and 0.4 weight percent talc nucleating agent.
The above cited U.S. Pat. No. 5,911,023 discloses optical fiber components, such as a buffer tube, a polymeric jacket or a slotted core, made of a thermoplastic polyolefin, preferably a propylene or ethylene homopolymer or a propylene-ethylene copolymer, characterized by a Melt Flow Index (MFI) higher than 3 and wherein a nucleating agent is disbursed therein, which result in substantial improvements in buffer tube crystallinity and crystallization rates, as well as reduced post extrusion shrinkage and improved crush resistance, gel compatibility and fiber length control.
U.S. Pat. No. 5,751,880 relates to an optical unit for an optical fiber telecommunications cable, the unit comprising a tube of plastics material, preferably polyethylene, polypropylene or polyvinyl chloride, having a modulus of elasticity less than 1500 MPa at 20° C. and a stress/elongation curve without a yield point. The proposed materials provide flexible and elastic tubes for the optical fiber unit, thereby making handling thereof easier. Because of its flexibility, the optical unit has greatly reduced “memory” thereby facilitating splicing operations on stranded loose tube structure cables.
EP-A-1,024,382 discloses a telecommunications cable element having a transmission element disposed in a buffer tube made from thermoplastic polyolefin elastomeric buffer material, preferably a propylene-ethylene copolymer, having a modulus of elasticity below about 500 MPa at room temperature and below about 1500 MPa at −40° C. Preferably, the polyolefin has an elongation at break below about 500% and an MFI above about 3. Flexibility of the elastomeric material is maintained over a wide range of temperature; further, its compatibility with low cost, thixotropic, water-blocking gel filling materials is observed.
International patent application publ. No. WO 01/09658 discloses a submarine optical cable comprising a polymeric buffer tube, specifically a polyester, e.g. polybutyleneterephtalate, a polyolefin, e.g. an ethylene or propylene homopolymer or an ethylene-propylene copolymer, or a polyamide, and a substantially non-deformable and hermetic elongated hollow body wherein at least one reinforcing element longitudinally extending along the whole length of the tube is embedded in the peripheral wall of said buffer tube.
The Applicant has however observed that the polyolefin materials used heretofore for manufacturing the optical buffer tubes may cause undesirable variations in the shape of the extruded buffer tubes. In particular, it has been observed that it may result difficult to maintain the desired substantially circular shape of the extruded buffer tube. After extrusion, the buffer tube is in fact subjected to a cooling step, typically by passing the tube along a water-cooling trough. As observed by the Applicant, a possible non-homogeneous (or differential) cooling of the tube (e.g. because of a variation in the thickness of the tube walls or because of the different temperatures between the polymeric melt forming the tube and the filling material and/or optical fibers contained therein), may determine an unwanted and uncontrollable variation of the desired shape of the tube. While this shape variation may be caused on any extruded component of an optical fiber cable which is subjected to a differential cooling, it becomes much more apparent when said element is a buffer tube, particularly having a non-symmetric design, for instance a buffer tube having a single strength member embedded into the peripheral wall of the buffer tube, such as the one disclosed in the above cited WO 01/09658.